Radiance Comparison from OCO-3 and OCO-2 Simultaneous Nadir and Ocean Glint Overpasses

The Orbiting Carbon Observatory 3 (OCO-3) was launched to the International Space Station (ISS) in May 2019 and has been making routine measurements since August 2019. OCO-3 records reflected sunlight in the near-InfraRed spectral region to estimate the column average CO2 dry air mole fraction (XCO2) and solar-induced chlorophyll fluorescence (SIF). Originally built as the spare copy of the OCO-2 flight instrument, OCO-3 shares most hardware characteristics with the OCO-2 spectrometer. Aside from an added polarizer and a telescope with a smaller blur spot, major differences are limited to external modifications like the pointing mirror assembly that was added to OCO-3 to accommodate operation on the ISS. An important difference of instrument operations between OCO-2 and OCO-3 is that, unlike OCO-2, OCO-3 is unable to perform direct solar observations, a limitation that is imposed by OCO-3s installation location on the ISS. This has consequences for in-flight radiometric calibration. In addition to pre-flight thermal vacuum testing, on-board calibration lamps, and lunar observations, OCO-3 relies on vicarious- and cross-calibration, including measurements over well-characterized surface sites and radiometric comparison with independent satellite sensors. The similarity of the OCO-2 and OCO-3 spectrometers makes OCO-2 the ideal candidate for cross-comparison of continuum radiances to establish radiometric accuracy. By mid-2021, about 200 incidences of OCO-2/3 near-Simultaneous Nadir/Glint Observations (SNOs/SGOs) i.e., both instruments recording measurements in nadir/glint mode over the same position on the Earth within 10 minutes of each other have been identified (this increases to about 600 cases for a relaxed time window of 30 min). SNO/SGOs occur at all latitudes but SNOs tend to be concentrated around 50 N/S, where the ISS ground track changes between ascending and descending node and the OCO-2 and OCO-3 ground tracks are thus at their largest relative angle. We report on how we determine and select SNO/SGO incidents, present results from radiometric comparisons of co-located OCO-2/3 observations for different surface conditions and terrain inhomogeneity, and show how SNO/SGO can help us in optimizing radiometric calibration for the OCO-3 instrument. Figure Caption: Continuum O2 A band radiances from OCO-2 (red) and OCO-3 (teal) for a Simultaneous Nadir Overpass over the upper Nile river. The significantly narrower swath of OCO-2 is due to the orientation of the OCO-2 spacecraft at the time the measurement was taken.